Philip Warner wrote:
>
> But I prefer the UDP/Collector model anyway; it gives use greater
> flexibility + the ability to keep stats past backend termination, and,as
> you say, removes any possible locking requirements from the backends.
OK, did some tests...
The postmaster can create a SOCK_DGRAM socket at startup and bind(2) it to "127.0.0.1:0", what causes the kernel
toassign a non-privileged port number that then can be read with getsockname(2). No other process can have a
socket with the same port number for the lifetime of the postmaster.
If the socket get's ready, it'll read one backend message from it with recvfrom(2). The fromaddr
must be "127.0.0.1:xxx" where xxx is the port number the kernel assigned to the above socket. Yes, this
is his own one, shared with postmaster and all backends. So both, the postmaster and the backends can
use this one UDP socket, which the backends inherit on fork(2), to send messages to the collector. If such a
UDP packet really came from a process other than the postmaster or a backend, well then the sysadmin has a
more severe problem than manipulated DB runtime statistics :-)
Running a 500MHz P-III, 192MB, RedHat 6.1 Linux 2.2.17 here, I've been able to loose no single message during the
parallel regression test, if each backend sends one 1K sized message per query executed, and the collector simply
sucks them out of the socket. Message losses start if the collector does a per message idle loop like this:
for (i=0,sum=0;i<250000;i++,sum+=1);
Uh - not much time to spend if the statistics should at least be half accurate. And it would become worse in SMP
systems. So that was a nifty idea, but I think it'd cause much more statistic losses than I assumed at first.
Back to drawing board. Maybe a SYS-V message queue can serve?
Jan
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